The present invention pertains generally to communications systems. More particularly, the present invention pertains to wireless communications links between line-of-sight stations in a communications system. The present invention is particularly, but not exclusively, useful as a communications link that can alternate between a laser mode and a microwave mode of operation in order to maintain effective communications in a wide variety of atmospheric conditions.
Wireless communications systems per se are well known in the prior art and have been used for communications links for the transmission of data, video and audio signals. Wireless systems offer the general advantage that they include links which do not require the laying of cables between the stations and, therefore, are more flexible in their installation and reconfiguration. One type of wireless system that is finding many new applications is a system which carries its communications signals on light beams.
Laser data links between line-of-sight stations are capable of handling high data transmission rates, which are in the range of several Gigabits per second (Gb/s). Laser data links, however, can be adversely affected by certain types of atmospheric conditions. For instance, it can be shown that haze, fog or heavy snow conditions will cause severe attenuation of the laser beam. This attenuation is due to scattering and when it happens, the laser data link becomes unreliable. Thus, even if adverse weather conditions are infrequent, an additional backup system may be necessary or desirable.
Microwave communications systems, like laser systems, are also well known as a means for establishing a wireless data link. Microwave data links, however, also have certain shortcomings. These shortcomings, however, are different than those associated with laser links. Specifically, microwave data links generally have a slower data transmission rate than laser systems. Typically, rather than transmitting data at rates of Gigabits per second (Gb/s), the data transmission rate for a microwave data link is less than a few hundred Megabits per second (Mb/s). Further, certain atmospheric conditions also adversely affect a transmitted microwave.
It happens that the atmospheric conditions which adversely affect microwave transmissions are different from the atmospheric conditions which adversely affect laser transmissions. Heavy rain, for instance, may not greatly affect a laser beam but it can cause significant attenuation of a microwave beam. On the other hand, it happens that a microwave beam is relatively unaffected by the same conditions mentioned above that will severely attenuate a laser beam, such as haze, fog and heavy snow. In sum, microwave data links can be reliably used to transfer data during periods when atmospheric conditions make a laser data link unreliable, and vice versa.
In light of the above, it is an object of the present invention to provide a hybrid wireless communications system that uses a single receiver for receiving either a laser beam or a microwave communications beam. It is another object of the present invention to provide a dual laser/microwave mode communications system that is available for effective data transfer between line-of-sight stations in a variety of atmospheric conditions. Another object of the present invention is to provide a dual laser/microwave mode communications system that is compact and space efficient. A further object of the present invention is to provide a dual laser/microwave mode communications system that is effectively easy to use, relatively simple to manufacture and comparatively cost effective.
A transceiver for a dual mode laser/microwave communications system in accordance with the present invention includes a housing which is formed with a single aperture for establishing communications along a common beam path inside the housing. As intended for the present invention, communications signals can be carried on either a microwave beam or a laser beam. In either case, the beam will be directed along the common beam path inside the housing from the aperture to a receiver (an incoming beam), or from an appropriate transmitter toward the aperture (outgoing beam).
Mounted within the housing of the system of the present invention are: an optical transceiver for receiving and transmitting communications on a laser beam; a microwave feed for receiving and transmitting communications on a microwave beam; a dielectric mirror (e.g. a beam splitter) for connecting either the optical transceiver or the microwave feed with a common beam path; and, a turning mirror for directing communications along the common beam path between the housing aperture and the dielectric mirror (beam splitter). In the preferred embodiment of the present invention, the dielectric mirror (beam splitter) is made of a material that will reflect a laser beam but which will allow the passage of a microwave beam through the mirror (beam splitter). Alternatively, the dielectric mirror will reflect a microwave beam and allow the passage of a laser beam. The turning mirror may be spherical or parabolic (or any other desired shape) and is preferably made of a metalized material which will reflect both a laser beam and a microwave beam.
For a preferred embodiment of the present invention, the turning mirror is formed with an opening at its center, and its reflective surface is oriented to establish the common beam path between the housing aperture and the dielectric mirror (beam splitter). The dielectric mirror (beam splitter) is then oriented to reflect either the laser beam or the microwave beam between the reflective surface of the turning mirror and the opening in the turning mirror. For this configuration, wherein the turning mirror has a central opening, a plurality of lenses and the optical transceiver can be positioned in the opening of the turning mirror for transmitting or receiving communications signals that are carried on a laser beam. It is, or course, also possible to mount separate laser transmitters on the outside of the housing. In this case, the optical transceiver will function only as a receiver. If microwave beams are directed toward the opening in the turning mirror, a microwave feed can be positioned in the opening of the turning mirror.
In an alternate embodiment, there is no need for the turning mirror to be formed with an opening at its center. Specifically, if the optical transceiver has a low profile configuration, such as is the case with a transceiver which incorporates an optical fiber, the optical transceiver (fiber) can actually be positioned between the turning mirror and the dielectric mirror (beam splitter). For either embodiment (i.e. the turning mirror with or without the central opening) the cooperation and combination of components for the system of the present invention is able to receive and to transmit communications signals on a laser beam.
Due to the use of a dielectric mirror (beam splitter) in the system of the present invention, a separate operation for receiving and transmitting communication signals that are carried on a microwave beam is also possible. Specifically, if the communications signal is carried on a microwave beam, the microwave beam will reflect from the turning mirror and then be reflected or passed by the dielectric mirror (beam splitter). This, of course, will depend on whether the dielectric mirror passes or reflects a laser beam and, in either case, it will do the opposite for the microwave beam. Thus, a microwave feed can be positioned within the housing behind the dielectric mirror for the reception and transmission of microwave beams. Preferably, the microwave feed is conically shaped and is sized for optimum reception or transmission of microwave beams in the range of thirty to fifty Gigahertz (30 GHz less than f less than 50 GHz).
For the operation of the dual mode laser/microwave system of the present invention in the laser mode, the optical transceiver at one station transmits a laser beam along a line-of-sight path. This laser beam is then received as an incoming laser beam by the optical transceiver at another station. At the receiving station this incoming laser beam is received through the aperture of the housing and is reflected by the turning mirror toward the dielectric mirror (beam splitter). The dielectric mirror (beam splitter) then reflects the laser beam towards the receiving optical transceiver. Depending on the particular embodiment of the system that is being used, the receiving optical transceiver may be located in an opening in the center of the turning mirror, or at some other location.
For the operation of the dual laser/microwave mode communications system of the present invention in the microwave mode, the microwave feed of the transceiver at the sending station transmits a microwave beam. This transmitted microwave beam passes freely through the dielectric mirror (beam splitter) and is reflected from the turning mirror toward, and through, the aperture of the housing at the sending station. This transmitted microwave beam is then received as an incoming microwave beam at the receiving station. At the receiving station, the incoming microwave beam is received through the housing aperture and is reflected from the turning mirror toward the dielectric mirror (beam splitter). Unlike a laser beam, however, the microwave beam passes through the dielectric mirror (beam splitter) and is received by the microwave feed. In this manner, the communications system of the present invention establishes a dual laser/microwave mode communications link between line-of-sight stations using a single aperture and a common beam path, for increased reliability over a single laser mode or microwave mode system, and increased data rate over a single microwave mode system. As indicated above, depending on the capabilities of the beam splitter, the respective positions of the laser beam optics and the microwave feed may be reversed.